The present invention relates to a preserving agent, a method and a container for maintaining the freshness of fresh marine products and particularly, preventing the change of color thereof when they are kept in cold storage in the raw state.
The color of the body of a marine product generally changes rapidly after the death. Since the change of color is related to the loss of freshness and leads to the degradation of the commercial value, prevention of change of color of marine products is an important problem to be solved. Change of color occurs in various marine products. They include crustaceans such as lobster, shrimp, prawn and crab, what is called red body fishes such as red sea bream (Madai), red rockfish (Menuke), bluefin searobin (Hobo) and alfonsino (Kinmedai), what is called white meat fishes such as cod and flatfish, tuna meat, bonito meat, sardine, mackerel, squid, cuttlefish and scallop.
For example, the colors of raw crustaceans such as lobster, shrimp, prawn and crab are apt to be changed into blackish colors soon after fishing, whereby the commercial value is greatly lowered. The portions at which the color is changed into blackish color is mainly the head portion in the case of lobster, shrimp or prawn and leg joint portions in the case of crab. The color is changed into blackish color because tyrosine in the body finally produces melanine polymer by the action of an oxidizing enzyme tyrosinase and the like in the presence of oxygen. It is therefore possible to prevent the color from changing into blackish color either by inactivating the enzyme by heating or by depressing the action of the enzyme by freezing. In the case of distributing crustaceans in the raw state, however, these methods cannot naturally be adopted. As a method of preventing the color of crustaceans from changing into blackish color in the raw state, if it is assumed to be one, a method of immersing crustaceans in a solution of an organic acid such as citric acid by utilizing the fact that tyrosinase is inactivated at a pH of not more than 5 is known. A method of immersing crustaceans in an aqueous sodium hydrogensulfite solution is also known. Both of these methods have insufficient effects and particularly the latter method has a problem in the regulation of residual SO.sub.2. In fact, it is the present state of art that there is no effective method other than shortening the distribution period from fishermen to consumers.
The red color of red fishes such as red sea bream is derived from red carotenoid pigments. Carotenoids are easily oxidized, thereby fading in color. As a countermeasure, red sea bream is immersed in a solution of reductive sodium ascorbate or an antioxidant is added to red sea bream. However, treatment with chemicals is sometimes undesirable and the effect thereof cannot be said sufficient.
In some marine products such as tuna meat, myoglobin which presents scarlet color is oxidized and changed into methomyoglobin which presents brown color. Fishes such as sardine and mackerel change the color into brownish color by browned substances which are formed when the lipid in the fish meat is oxidized.
Since the change of color in fresh marine products is often caused by oxidation, as described above, use of a deoxidant has been proposed. However, not only low deoxidizing ability of conventional deoxidants at a low temperature under the cold-storage conditions but also a conventional method of merely removing oxygen still has a problem from the point of view of the total maintenance of the freshness although the change of color can be prevented. Thus, the improvement on maintaining the freshness of marine products has been demanded.
As described above, in the case of distributing fresh marine products in the raw state, the limitation of time for maintaining the freshness is one or two days even they are kept in cold storage, and if the time exceeds the limitation, the color changes. Therefore, if there is some trouble in transportation on the way, the commercial value is greatly lowered. Transportation from remote districts which require a long time is often impossible in the present state of art.
Accordingly, it is an object of the present invention to solve the above-described problems in the prior art and to provide a preserving agent, a method and a container for preventing the freshness of fresh marine products from lowering and, in particular, preventing the change of color of the body thereof.